Depends on the team.
If they're young and have a long way to max their categories, I tend to practice less... (20/20)
If I have a bunch of studs near maxed individually, I go up to 28/28. This is extreme and wouldn't recommend for most teams, but when I'm mostly maxed, that is what will make the difference.
Know your team. And go typically between 20-25 with rare exceptions (all freshmen w potential might require 15 at first)